Experimental Leishmania major infections in mice have been used extensively to understand how cell-mediated immunity (CMI) develops, and to specifically define the factors that dictate whether a Th1 or Th2 response is observed after activation of T cells. Such studies have clearly shown an important role for IL-12 in the development of resistance and a Th1 response. However, despite a great increase in our knowledge of the events that are associated with the development of Th1 responses, little is understood about the rules that govern the maintenance of CMI. Our laboratory has recently shown that IL- 12 is required not only to initiate Thi cell development, but also to maintain this response. This proposal seeks to determine how IL-12 participates in maintaining CMI, and in so doing will more broadly investigate how immunologic memory works in L. major healed mice. Our specific aims address the three critical components for CMI: memory T cell function (Aim 1), the antigen-in this case the role of parasite persistence (Aim 2), and the accessory cells-specifically dendritic cells-that both present antigen and influence the nature of the T cells that develop (Aim 3). The working hypothesis of this proposal is that CMI requires the constant renewal of the Thi population from a non-polarized pool of T cells. To test this hypothesis a series of adoptive transfer experiments are proposed, both with conventional T cells, as well as TCR transgenic T cells recognizing Leishmania and non-Leishmania antigens. The donor cells will be tracked in the recipient mice to assess their fate. An analysis of the role of parasite persistence will use a L. major (dhfr-ts-) thymidine auxotroph that infects mice, but fails to survive. Finally, the role of antigen presentation will be assessed by characterizing the dendritic cell response associated with resistance. Preliminary studies from this laboratory demonstrated that CD4O-CD4OL interactions are not required for immunity, and in this aim, the compensatory role of TRANCE will be tested. Overall, these experiments should provide a clear picture of the dynamic interactions between T cells, dendritic cells and persisting parasites that are required to maintain cell-mediated immunity.